




: " 5*S*r?4» 


DEPARTMENT OF COMMERCE 



OF THE 


i&^r' . •: 

I | l| '■■-- ■ ■■ 

fljlf 


Bureau of Standards 


mm 




mm , 

wt* ■ 


lii- 


: ;*• 



4 . 

i!t|: ’ r. 

•f. -■ 

>>'• • •• • • - V r 



3 «>J; 


SS 

;' 




li’i . > 




V r; 

- >.s - / 

'•:~V 


S. W. STRATTON, Director 


No. 335 


EFFECT OF RATE OF TEMPERATURE CHANGE 
ON THE TRANSFORMATIONS IN AN 
ALLOY STEEL 


BY 


HOWARD SCOTT, Assistant Physicist 

Bureau of Standards 


ISSUED JULY 10, 1919 


: 



PRICE, 5 CENTS 

Sold only by the Superintendent of Documents. Government Printing Office 

Washington, D. C. 


WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1919 


JSonogra ph 


• * - v , r 

Mg . , 


k '.- ■ 

. ... 


V- \ &*•' 

























DEPARTMENT OF COMMERCE 


Scientific Papers 

OF THE 

Bureau of Standards 

S. W. STRATTON, Director 


No. 335 

EFFECT OF RATE OF TEMPERATURE CHANGE 
ON THE TRANSFORMATIONS IN AN 
ALLOY STEEL 


HOWARD SCOTT, Assistant Physicist 

ii 

Bureau of Standards 


ISSUED JULY 10, 1919 



PRICE, 5 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 

Washington, D. C. 


WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1919 

















0» C * ■* * 
;UL .24 19)9 



1L 



EFFECT OF RATE OF TEMPERATURE CHANGE ON 
THE TRANSFORMATIONS IN AN ALLOY STEEL 


By Howard Scott 


CONTENTS 

I. Introduction. 

II. Previous investigations. 

III. Experimental method.. 

IV. Thermal curves. 

V. Effect of cooling rate. 

1 . Microstructure. 

2. Relation of Ar' to Ar // . 

3. Suppression of Ar 7 . 

VI. Transformations on heating. 

1. Transformation Acz. 

2. Effect of previous cooling rate on Ac i-j 

VII. Summary. 

I. INTRODUCTION 

Since Bohler discovered in 1903, on cooling certain alloy steels ? 
the phenomenon of a new and lower temperature transformation 
than the usual Ar 3-2-1 obtained by increasing the maximum 
temperature (T max.) to which the material was heated, a con¬ 
siderable amount of work has been published, 1 connecting this 
phenomenon with a large number of dissimilar steels of high alloy 
content. From the fact that the transformation divides itself, 
taking place at two widely separated temperatures, it has been 
called a split transformation. The significant facts established by 
recent investigators 2 are (a) that when the transformation occurs 
at the higher temperature, Ar', troostite or a decomposition 
product is formed and ( b ) that when the transformation occurs 
at the lower temperature, Ar", the resulting structure is marten¬ 
site. The terminology Ar' and Ar", adopted here, is that of 
Portevin. 3 

1 Yatseviteh, Rev. de Met., 15 , p. 65; 1918, Bibliography to 191s. 

s Dejean, Rev. de Met., 14 , p. 641: 1917. Portevin, ibid., 14 , p. 707; 1917. Edwards, J. Iron and Steel 
Inst., 93 , p. 114; 1916. 

3 Portevin, loc. cit. 

106425°—19 91 


Page 

91 

92 

93 
93 
96 

96 

97 

97 

98 

98 

99 

100 


















92 


Bulletin of the Bureau of Standards 

II. PREVIOUS INVESTIGATIONS 


[Vol. IS 


Reviewing the work on this subject 4 published in a recent 
issue of Revue de Metallurgie and referring in particular to his 
statement that martensite is a solution of carbide in alpha iron, 
H. Le Chatelier says: 

How then can a theory' already 20 years old demand new investigations? The 
reason for it is that we have not succeeded in proving directly the real presence of the 
transformation of iron diming the very short duration of the quenching. The fall of 
temperature takes place at the rate of several hundred degrees per second and the 
observation of phenomena so rapid requires particularly sensitive methods of record¬ 
ing. I have attempted without success to observe the moment of the reappearance 
of the magnetic property' during the quenching of bars 15 mm square, but the inequali¬ 
ties of temperature from one point to another in the mass conceal the phenomenon. 
M. Chevenard 5 , in working on wires of a diameter 100 times smaller and using as a 
characteristic of the transformation of iron the change of length instead of the varia¬ 
tion of magnetism, has surmounted for the first time the difficulties which seemed at 
first view insurmountable, and he has done it with an extreme precision. The thermal 
measurements of Portevin and Garvin 6 and of Dejean 7 lead to the same conclu¬ 
sions, although in a fashion less direct. 

The results presented here add further confirmation of the 
theory of Te Chatelier, though in a less direct manner than those 
of Chevenard. Thermal and microscopic data are brought for¬ 
ward here to establish the effect of rate of temperature change on 
the temperature and nature of the transformations in a steel of 
the composition C, 1.75; Mn, 0.26; Co, 2.90; Cr, 15.0. 

Similar work has been done by Edwards 8 on a steel of the 
composition C, 0.63; Cr, 6.15; Si, 0.07; Mn, 0.17. 

However, he arrives at the conclusion that— 

The maximum hardness was obtained when the thermal transformation had been 
entirely' prevented, and when this was accomplished the steel was purely martensitic 
in structure. 

The present work fails to confirm this statement, as does that 
of the investigators already referred to. Edwards was unable 
to observe the transformation Ar" with the formation of marten¬ 
site, probably for the reasons given by Rosenhain 9 in his dis¬ 
cussion of Edwards’ paper. 

Yatsevitch, 10 Dejean n , and Honda 12 have used two or three 
cooling rates in their experiments with varying T max., and their 
results show, as do Edwards’s that the transformation split occurs 
for lower values of T max. with faster cooling rates. 

4 Le Chatelier, Rev. de Met., 14 , p. 601; 1917. 3 Rosenhain, J. Iron and Steel Inst., 93 , p. 147; 1916. 

6 Chevenard, Rev. de Met., 14 , p. 610; 1917. 10 Yatsevitch, loc. cit. 

8 Portevin and Garvin, Rev. de Met., 14 , p. 607; 1917. 11 Dejean, loc. cit. 

T Dejean, Rev. deMet., 14 , p. 641; 1917. 15 Honda and Murakami, Sci. Rep. Tohoku Imp. 

8 Edwards, loc. cit. Univ., 6, p. 235; 1918. 




Bulletin Bureau of Standards, Vol. 15. 



106425°—19. (To follow page 92) J'lG. I.— Thermal curves of sample A 

























































> 

f 






































Scott ] 


93 


Transformations in an Alloy Steel 

The previous investigators in this field have laid particular 
stress on the variation of T max., the rate remaining constant, 
while the variation of rate, T max. remaining constant, has 
received little attention. The present work attempts to apply 
the latter method to the investigation of an alloy steel with the 
object in view of correlating the results of that method with those 
of the former and to establish the relationship between the several 
phenomena observed. 

III. EXPERIMENTAL METHOD 

The method employed for obtaining the thermal curves was 
to heat the samples, attached to the hot junction of a 0.5 mm 
diameter platinum, 90 platinum-10 rhodium thermocouple, in an 
electric vacuum furnace, taking potential measurements on a dial 
potentiometer and measuring the time interval on a chronograph, 
as described in the Bureau of Standards Scientific Paper 213. 
The furnace, however, was one recently built at the Bureau, 
a modified form of the one described by Rosenhain 13 and in use 
at the National Physical Laboratory. This furnace, which will 
be described at a later date, was admirable for the purpose at 
hand, as extreme rates of temperature change can be obtained 
with smooth curves over long ranges. 

IV. THERMAL CURVES 

The curves of Figs. 1 and 2 were plotted by the inverse rate 
method from readings taken every 0.02 millivolt (approximately 
2 0 C) except for several extremely fast runs, which, however, are 
plotted on that basis. 

The curves of Fig. 1 are a preliminary series taken on sample A 
of about 10 g mass to locate the transformation ranges and with¬ 
out fully knowing the characteristics of the material. The data 
for the curves of Fig. 2 were taken on sample B, mass 0.81 g, 
keeping T max. constant and extending th£ observations to lower 
temperatures than for sample A. The values given for rate of 
temperature change were reduced from the inverse rate curve 
observations taken on heating just before Ac i-j and on cooling 
midway between AN and Ar". 

The transformations as designated on the curves of Figs. 1 and 2 
are Act, an evolution of heat on heating a sample previously 
cooled at a rate that gave Ar"; Ac a, the magnetic transformation; 
Ac 1-3, the transformations Ac 1 and Ac 3 merged or nearly super¬ 
imposed; Ar', the upper transformation of the split Ar trans¬ 
formations; and Ar", the lower transformation. The temperature 
values of these transformations are collected in Table 1. 


13 Rosenhain, Inst, of Metals, 13 , p. 164; 1915. 




TABLE 1.—Temperature Ranges of Transformations in Degrees Centigrade 

SAMPLE A 


94 


Bulletin of the Bureau of Standards 


[Vol. is 


T max. 

1050 

1125 

1110 

970 

925 

880 

870 

925 

925 

< 

End 

395 


357 

00 

rH 

CO 

Maxi¬ 

mum 

414 


396 

385 

Begin¬ 

ning 

446 


422 

401 

* 

End 

• 

• 1-4 O 

• Cl TT 

• VO 

• 

• 

• 

670 

714 

734 


Maxi¬ 

mum 

735 

715 

760 

748 

766 


Begin¬ 

ning 

757 

742 


755 

772 

776 


Rate of 
cooling 
°C.per 
second 

0.18 

.25 

.05 

.50 

.28 

.13 

.04 

.80 

.71 

? 

q 

«c 

End 

856 

855 

841 

C>» VO ^ 

00 00 00 00 00 

Maxi¬ 

mum 

^OlO'OvOOlN HO' 
r< (m h n p-< o *-» eo 

C0CC0C0CCCCOO0CO30 

Begin¬ 

ning 

W) Oi 

CM H 

00 00 


804 

794 

809 

821 

809 

CM 

u 

< 

«-4 r-4 o CM p-4 O *00 • 

00 00 00 00 00 00 • t". . 

C' • 


•a 

a 

W 


■a a 

« 3 

SE 


t 3 M 
*bfl 9 
©a 
« 


.9 Sa 

"ti 01 • o 

fa jd o vj 


a 

3 

fa 


o o vo cc 

CM CO O O 


T3 • - • • £ r- 

c *a £ • ' c £ 

ici 2 § « S 


o o 

CM CM 

o\ o\ 


n 

G\ 


O 

CM 

a\ 



340 

371 

387 

392 


M O' 't CO VO O O • ( • 

O d CM O CO y* • * » 

CO sr n sj- Tf xf • • • 

* • • 

• • • 

• • • 


381 

429 

442 

449 


681 

681 

680 

700 

723 

738 


• VO y~< C^COO*—ivOnO 

• O *-4 r-4 Cl Tt- *—t »n CO VO 

• c>. t^voc^c^*r^c^c>. 

( a ) 

748? 

740 

748 

759 

775 

770 

3.40 

1.20 

.60 

.50 

.43 

.17 

.09 

.06 


840 

832 

832 

835 

829 

835 

842 


oo VO VO 00 00 •«- Cl 

r? V-* CM CM 

GO 00 00 00 00 00 00 


808 

809 

808 

813 

807 

814 

821 


780 

780 

779 

778 

778 

778 


644 

644 

628 

637 

628 


637 

634 

601 

621? 

601? 


592 

578 

544 

518 

564 


0.27 

.31 

.31 

.33 

.20 

.20 

.26 


*d 

a 

s 2 

•- 4 > 

fa 00 


<a 

H 


£3 

■c 

3 

o 

fa 


4 ) 

> 

4) 

CO 


43 

bfi 

3 


a Rate of cooling too fast for close observations. 















































































































Scott] 


Transformations in an Alloy Steel 


95 


The appropriateness of the transformation notation Ac 1-3 and 
Ac t will be seen from the discussion of those transformations. 



Two values for the maximum transformation temperature indicate 
a double peak. In Fig. 3 the temperature values of Ac 1-3, Ar' f 
and Ar" given in Table 1 are plotted against rate of temperatu±e 







































96 


Bulletin of the Bureau of Standards 


[Vd. 15 


change in degrees centigrade per second. No attempt is made to 
interpret the double peaks, and the lines representing Ar' and Ar" 
in Fig. 3 are rather arbitrarily drawn through the higher values. 

V. EFFECT OF COOLING RATE 

An inspection of the cooling curves of Figs. 1 and 2 shows that 
Ar' is the normal Ar 3-2-1 of slow cooling rates, but that it grad¬ 
ually dies off in intensity with increasing rate. While Ar' is falling 
off in intensity, the transformation Ar" comes into existence and 
gains in intensity, being a maximum for rates that do not show Ar'. 

This region over which both Ar' and Ar" occur, as shown in 
Fig. 3, will be called the critical cooling range. Its limits were 
roughly determined as 0.15 and 0.70° C per second, by plotting a 
measure of the transformation intensities, obtained by a method 
to be described in a subsequent section, against rate and extending 
a straight line through the values back to zero. 

The remarkable change in properties caused by this very slight 
change in rate is represented, when the same phenomenon is 
observed on varying T max., by the considerable temperature 
variation of approximately 300° C for some high-speed steels. 14 

The fact that the split transformation occurs with a constant 
value of T max. shows that it is unnecessary to hypothecate a 
dissociation of the carbide (or carbides) to explain this phenom¬ 
enon. 

1 . MICROSTRUCTURE 15 

To establish the structural difference between the material 
cooled at a rate that gave Ar' and one that gave Ar" and the anal¬ 
ogy to the phenomena obtained by varying T max. for this steel, 
micrographs were taken of samples cooled at several definite rates 
of cooling. The micrograph, Fig. 4, taken after cooling at a rate 
of o.oi° C per second, Ar' only occurring, shows an irregular mass 
of fine carbide particles, corresponding to pearlite in carbon steels 
and distinct from the coarse particles of free carbide, in a ferrite 
matrix. Figs. 5 and 6, micrographs, taken of samples cooled at 
rates of 0.30 and 0.33 0 C per second, respectively, show character¬ 
istic black troostite patches on a background of martensite. With 
those cooling rates the transformations Ar' and Ar" were both 
obtained. Fig. 7, which is of sample A following a cooling rate 
of 0.71 0 C per second, shows a martensitic structure although the 


14 Honda and Murakami, loc. cit.; Carpenter, loc. cit. 

15 Micrographs by H. S. Itawdon. 



Bulletin Bureau of Standards, Vol, 15 



Fig. 4. —Cooling rate, o.oi° C per second. Transformation A/ 
Magnification iooox. Etched in 2 per cent HNO 3 in alcohol 



Fig. 5 . —Cooling rate, 0.30° C. per second. Transformation A/ and AC' 
Magnification iooox. Etched in 2 per cent HNO3 in alcohol 



















Bulletin Bureau of Standards, Vol. 15 



Fig. 6 . —Cooling rate, o.jj 0 C per second. Transformation A/ and Ar" 
Magnification iooox. Etched in 2 per cent HNO3 in alcohol 



Fig. 7 . —Cooling rate, o.yi° C per second. Transformation Ar // 
Magnification iooox. Etched in 2 per cent HNO3 in alcohol 



























Scott] 


Transformations in an Alloy Steel 


97 


needlelike markings, characteristic of high-carbon steels, are only 
slightly evident. 

The conclusions to be drawn from the preceding microscopic 
evidence are that troostite or a decomposition product forms with 
the transformation Ar' and martensite with the transformation 
Ar", precisely what obtains when the same transformations are 
observed in other alloy steels with varying T max. 

2 . RELATION OF “Ar"’ TO “Ar"” 

The radical structural difference between the material showing 
Ar' and that showing Ar" presumes a similar radical difference 
in the transformations Ar' and Ar". To demonstrate the possi¬ 
bility of this difference, the intensities of the transformations 
Acr-j, Ac 2, Ar', and Ar" have been estimated by means of a 
planimeter measuring the area of the positive departure of the 
thermal curves from the assumed neutral body curves through the 
respective transformation ranges. The results given in Table 2 
show a well-marked loss in intensity of the sum of the areas of Ar' 
and Ar" at the cooling rate 1.20 0 C per second, which gives Ar" 
above. On the assumption that Ar'' is no new transformation other 
than Arj, 2, or 1, the conclusion is that some one or more of the 
transformations Arj, 2, and 1 constituting Ar' is suppressed. 


TABLE 2.—Areas of Thermal Curves in Square Millimeters Corresponding to Hea 

Effects of Transformations in Sample B 


Run 


First.. 

Second.. 

Third. 

Fourth. 

Fifth. 

Sixth. 

Seventh. 

Eighth. 

Average. 


Acl-3 


36 

40 

40 

64 

60 

72 

72 


Ac# 


74 

66 

68 

64 

64 

72 


68 


Ac7-S+ 

transfor- 

mation 

Ac# 


110 

106 

108 

128 

124 

144 


120 


Cooling 
rate, de¬ 
grees per 
second 


3.40 
1.20 
.60 
.50 
.43 
.17 
.09 
.06 


Ar' 


26 

38 

56 

100 

120 

116 


Ar" 


84 

72 

60 

48 


Ar' trans¬ 
forma¬ 
tion H-Ar" 


84 

98 

98 

104 

100 

120 

116 


3 . SUPPRESSION OF “ A11 ” 

The preceding conclusion agrees with the generally accepted 
conception that martensite is a solid solution of cementite in some 
form of iron. This means that Arj is suppressed with the for¬ 
mation of martensite and further evidence is not wanting. The 
transformation intensities indicate that a heat effect of the mag- 

































98 Bulletin of the Bureau of Standards [ voi. 15 

nitude of Ar 1 is missing at Ar". The work of Honda on the mag¬ 
netic properties of tungsten steels in a paper before the Septem¬ 
ber meeting of the Journal of the Iron and Steel Institute, how¬ 
ever, shows that the carbide is retained in solution at Ar" for the 
carbide in solution does not undergo the transformation Ao. The 
transformations Ac t and Ac7-5 offer still further substantiation 
to which attention will be called in their discussion. 

There still remains the possibility that one of the other trans¬ 
formations, Arj or 2, is suppressed. This, however, is mani¬ 
festly impossible, for Aj and A 2 coincide when Aj is depressed 
below the normal temperature of A 2 18 and martensite is mag¬ 
netic. The magnetic curves of Honda and Murakami 17 , taken 
on a number of tungsten steels showing a split transformation with 
increasing T max., also indicate the occurrence of A 2 at Ar". 

The conclusion that must therefore be adopted is that Arz is 
suppressed with the formation of martensite, or that Ar" con¬ 
stitutes the transformations Arj and 2. 

VI. TRANSFORMATIONS ON HEATING 

The thermal curves of Figs. 1 and 2 show two transformations 
Ac 1-3 and Ac 2 occurring uniformly within narrow temperature 
limits and a transformation Ac/ occurring only following certain 
cooling rates. The identity of Ac 2 is established by its markedly 
characteristic shape and its uniform occurrence at about 780° C 
which is in close proximity to its maximum, 768°, in pure iron. 
This phenomenon of Ac 1 occurring above Ac 2 in alloy steels is 
not new and has been well established by Moore 18 for a chromium 
steel. The transformation Ac 1-3 hardly needs identification, 
although attention should be called to its sluggish ending which 
indicates that the transformations concerned do not completely 
coincide. This is further illustrated by the change in area of the 
peak, which is evidently Acz from the effect of previous cooling 
rate on its position, with the temperature of its occurrence. 

1 . TRANSFORMATION “Ac t” 

The transformation Act is indicated by an inflection to the left 
which denotes an evolution of heat on the heating curve and occurs 
over a considerable temperature range. It is a maximum follow¬ 
ing cooling rates that give Ar" alone, and loses in intensity fol¬ 
lowing decreasing rates through the critical cooling range, becom- 


16 Honda and Takagi, Sci. Rep. Tohoku Imp. Univ., 6 , p. 324 ; 1918 . 

17 Honda and Murakami, loc. cit. 

18 Moore, J. Iron and Steel Inst., 81, p. 268 ; 1910 . 



I 


Scott) Transformations in an Alloy Steel 99 

ing zero when Ar' alone occurs. It is therefore roughly propor¬ 
tional in intensity to Ar" or the amount of martensite present. 
By its analogy to tempering the conclusion may be drawn that 
Ac t represents the precipitation of the carbide in solution to form 
at first troostite and as it progresses the coarsening of the carbide. 

This phenomenon of a heat evolution on heating steels that 
show Ar" was observed by Carpenter 19 on differential thermal 
curves with which T max. was varied and connected with tem¬ 
pering. 

The nature of Ac/, a gradual building up of the heat evolution 
over a long temperature range, may throw some light on the 
spontaneous heat evolution and also the change in other physical 
properties of quenched steels as observed by Hadfield and Brush 20 , 
by Matsushita 21 , and by Campbell 22 . The indications are that 
the transformation starts to a minute degree at very low temper¬ 
atures, possibly at ordinary temperatures, particularly in carbon- 
steels which temper at lower temperatures than alloy steels. 

The existence of Ac t as an evolution of heat following cooling 
rates that give Ar" is further confirmation of the suppression of 
Arj with the formation of martensite. 

2 . EFFECT OF PREVIOUS COOLING RATE ON “Ac 1 - 8 *' 

It will be seen on examining Fig. 3 that practically all the tem¬ 
perature values for the maximum of Ac 1-3 lie on two smooth 
curves. The data of Table 1 show that the runs which correspond 
to the numbers on the upper curve were obtained following cool¬ 
ing rates that gave Ar' predominant and those on the lower curve 
following cooling rates that gave Ar" predominant. The tem¬ 
perature interval, io° to 15 0 C, between those two curves may 
therefore be attributed to the state of division of the carbide 
resulting from the previous heat treatment. 

The phenomenon noted in the preceding paragraph offers still 
further substantiation of the suppression of Ar' with the forma¬ 
tion of martensite. 

It may be of interest to note that the curves of Fig. 3 drawn 
through the temperature values of Acr-j and Ar' do not point 
toward a common equilibrium temperature Aei. 

19 Carpenter, J. Iron and Steel Inst., (57, p. 433 ; 190 ?. 

20 Hadfield and Brush, Proc. Royal Soc., 83, p. 188 ; 1917 . 

21 Matsushita, Sci. Rep. Tohoku Imp. Univ., 7, p. 43 ; 1918 . 

22 Campbell, Reprint J. Iron and Steel Inst., 98,p. 421 ; 1918 . 




ioo Bulletin of the Bureau of Standards lVoi. 15 

VII. SUMMARY 

The results of previous investigators have been taken to show 
that with the occurrence of a split transformation on cooling alloy 
steels from increasingly higher temperatures (a) that when the 
higher temperature transformation Ar' is observed with low 
values of T max., troostite or a decomposition product results 
and ( b ) that when the lower temperature transformation Ar" is 
observed with high values of T max., martensite is the resulting 
product. 

The present investigation has shown for a certain alloy steel 
that on varying the rate of cooling, the maximum temperature 
remaining constant, a strictly analogous phenomenon is observed, 
increasing rate of cooling having the same effect as increasing 
T max. 

v. 

Conclusions are drawn to the effect that— 

(а) The transformation Ar' consists of the transformations 
Ary, 2, and 1. 

(б) The transformation Ar" consists of the transformations 
Ary and 2. 

(c) The transformation Ar 1, suppressed when Ar" is observed, 
occurs on heating as Ac t with an evolution of heat and the forma¬ 
tion of troostite or a coarser condition of the carbide. 

(< d ) The maximum of the transformation Ac j-y occurs at a 
higher temperature when the previous cooling rate gave Ar' than 
when it gave Ar". 

The author desires to express his indebtedness to H. S. Rawdon 
for the micrographic work and to Miss P. L. Thompson for her 
skillful assistance in preparing the experimental data. 

Washington, December 23, 1918. 










‘ 















































































■ 














■ 






■ 


















































































































